Neutrons are subatomic particles with no net electric charge. Neutrons and protons, another subatomic particle, together form the nucleus of all elements in the periodic table except hydrogen. Free neutrons are produced as a consequence of either nuclear fission, radioactive decay of elements or fusion. Special nuclear materials (“SNMs”) such as plutonium that are used for making dirty bombs decay radioactively to produce neutrons. Detection of such neutrons is an effective way of tracking the source of SNMs. However, since neutrons do not carry any electric charge, their detection is problematic as compared to other charged subatomic particles. One method of neutron detection that has been successfully employed is to use materials that can capture incident neutrons and convert them into other easily detectable subatomic particles, such as alpha particles, tritons, gamma rays, etc.
Historically, high-pressure Helium-3 (He3) tubes have been the mainstay of neutron detection. Neutrons impinging on these tubes interact with He3 nuclei to produce triton and protium, both of which are energetically charged subatomic particles that migrate in the presence of a strong electric field inside the tubes towards the electrodes. Unfortunately, He3 supplies on the planet are running low and the price of He3 in recent years has increased twenty-fold in the last decade alone. Thus, there is a strong consensus in the field to replace He3 technology with alternatives, mostly scintillation based detection systems and Boron lined proportional tubes.
Scintillator detectors also have several limitations. First, scintillation crystals are expensive and made in small volumes due to a limited market. Second, complicated pulse shape discrimination algorithms need to be employed in these systems to discriminate neutrons from gamma rays, which also interact heavily with the scintillating crystals. Scintillation detectors also suffer from reliability issues on the field due to the use of scintillating crystals that can be sensitive to environmental factors such as humidity and salinity. The gamma discrimination capability of Boron lined tubes is better than scintillator detectors. However, being a proportional counter technology, Boron lined tubes are limited by their form factor in the scope of their applications. Moreover, there is an absence of a global supply chain to drive down their cost over time. Both scintillation and proportional counter based systems must contend with significant system-level noise that interferes with measurements of low incident neutron flux levels close to the cosmic background. They also lack modularity, flexibility to detect subatomic particles other than neutrons, and potential for rapid scalability.